INTRODUCTION

It is known that, for the targeted improvement of the properties of polypropylene (PP), the modification method is widely used, which consists in the creation of polymer-polymer compositions [1].

Isotactic polypropylene (PP) is often used to create polymer-polymer compositions with improved properties on its basis. However, PP, high pressure polyethylene (PE), and many other polymers are not compatible with each other. To improve their compatibility, it is necessary to have functional groups in their composition or to introduce compatibilizers or nanofillers (NF) into the composition as interfacial additives that improve both the compatibility of the components and the operational properties of the obtained materials [2–8].

The use of dispersed nanofillers makes it possible to control the structure and properties of materials through nucleation and orientation effects, changes in the conformation of macromolecules, their chemical bonding with the surface of nanoparticles, and “healing” of structural defects [9–11].

The development of research on nanosized and cluster metal-containing particles in polymer matrices was largely facilitated by the creation of metal-polymer composite materials with specific physical, mechanical, and operational properties: increased thermal and electrical conductivity, high magnetic susceptibility, the ability to screen ionizing radiation, etc. [12, 13].

It is known that the use of nanoparticles (NPs) of d-valence metals (copper, zinc, cobalt, nickel, etc.) in polymers makes it possible to obtain fundamentally new materials that are widely used in radioelectronics and optoelectronics as magnetic, conductive, and optical media [14, 15].

The aim of this work is to study the effect of small NF additives containing NPs of metal oxides on the physicomechanical, rheological, and thermal properties of composites based on isotactic PP and PE.

EXPERIMENTAL

In this work, we used the following:

—isotactic PP Kaplen (Russia) brand 01 030 with a molecular weight of ~2–3 × 105, with a polydispersity index of 4.5, with a melt flow rate (MFR) of 2.3–3.6 g/10 min;

—high-pressure polyethylene grade 15803-020 (PE), ρ = 0.917–0.921 g cm–3, MFI 1.5–2.5 g/10 min.

As a nanofiller, we used NPs of zinc oxides stabilized by a polymer matrix of high-pressure polyethylene obtained by a mechanochemical method in a polymer melt. The content of nanoparticles was 5 wt %, the size was 39 ± 1.0 nm, and the degree of crystallinity was 35–45% [16]. The ratio of the co-mponents of the composition (in wt %) was PP/PE/NN = 50/50/(0.5, 1.0, 2.0) Nanocomposite polymer materials were obtained by mixing PP and PE with zinc-containing NF on laboratory rollers at a temperature of 160–165°C for 15 min. For mechanical tests, the resulting mixtures were pressed in the form of plates 1 mm thick at 190°C and a pressure of 10 MPa for 10 min.

The physicomechanical parameters of the obtained compositions were determined on an RMI-250 device, the melt flow rate (MFR) was obtained on a CEASTMF50 capillary rheometer manufactured by INSTRON (Italy) at a temperature of 190°C and a load of 5 kg. X-ray phase analysis (XRD) of the obtained compositions was carried out on a D2 Phaser instrument from Bruker (Germany). The thermal stability of the studied samples of nanocomposites was studied on a Q-1500D derivatograph (MOM, Hungary). The tests were carried out in an air atmosphere in a dynamic mode where the sample was heated at a rate of 5 K min–1 from 20 to 500°С, a weighed portion of 100 mg, the sensitivity of the channels of differential thermal analysis (DTA) of 250 μV, thermogravimetry (TG) of 100 mV, differential thermogravimetric analysis (DTG) of 1 mV.

RESULTS AND DISCUSSION

The effect of NF containing zinc oxide NPs on the structure of a composite based on PP/PE was investigated by the X-ray phase method. Figures 1 and 2 show the XRD patterns of the initial PP/PE and PP/PE with a zinc-containing nanofiller. Reflections corresponding to the initial PP/PE (Fig. 1) and reflections characteristic of zinc-containing nanoparticles are shown: dhkl 2.47966, 2.13735, 1.51401, 1.28859 Å (Fig. 2), which corresponds to the dhkl series of zinc oxide according to the ASTM card index. [d-Spacings (20)—01-071-3645 (Fixed Slit Intensity)—Cu Kα1 1.54056 Å. Entry Date: 11/19/2008. Last Modification Date: 01/19/2011]. Table 1 shows the physicomechanical and rheological parameters of composites based on PP/PE containing a nanofiller with zinc oxide nanoparticles.

Fig.1.
figure 1

Diffraction pattern of the sample PP/PE.

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Fig. 2.
figure 2

Diffraction pattern of the sample PP/PE/NF.

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Table 1. Physicomechanical and rheological parameters of the obtained nanocomposites
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As can be seen from the data in the Table 1, the introduction into the composition of 0.5 wt % NF leads to an increase in the strength index from 13.15 to 15.27 MPa. An increase in the concentration of NF greater than 1.0 wt % leads to a decrease in the strength of the composite (13.93 MPa), which is probably due to the aggregation of nanoparticles, which leads to the formation of microdefects in the bulk of the polymer matrix. Introduction to the composition of 0.5–1.0 wt % NF leads to an increase in the deformation value upon fracture of the composite by a factor of 1.2, which is apparently associated with the synergistic effect of the interfacial interaction of zinc-containing nanoparticles in the PE matrix with the components of the PP/PE polymer composition, the mutual influence of which contributes to an increase in both the deformation value and the strength index.

The study of the Vicat heat resistance of the obtained compositions showed that the introduction of a nanofiller into the PP/PE composition hardly affects the heat resistance index.

At the same time, an increase in the content of nanofiller (1.0–2.0 wt %) promotes an increase in the melt flow rate (MFR) to 19.7 (1.0 wt %) and 34.6 (2.0 wt %) g/10 min, which indicates an improvement in the fluidity of the composition and the possibility of processing it by injection molding and extrusion.

The thermal stability of the studied samples based on PP/PE containing NFs with zinc oxide NPs was estimated from the value of the activation energy (Eа) of thermo-oxidative destruction, calculated by the method of double logarithm of the TG curve according to the method described in [17], at a temperature of 10% (T10), 20% (T20), and 50% (T50) decay of the samples under study, as well as their half-life—τ1/2. The results of derivatographic studies are shown in Table 2.

Table 2. Thermal properties of the studied samples of nanocomposites
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It has been shown that the introduction of zinc oxide NPs into the composition of the compound promotes a significant increase in the decomposition temperature of the samples: T10—from 200 to 335°C, T20—from 275 to 355°C, T50—from 325 to 390°C; the half-life τ1/2 increases from 55.6 to 74.9 min; the activation energy (Eа) of thermo-oxidative destruction of the obtained nanocomposites increases from 191.4 to 248.5 kJ/mol.

As a result of derivatographic studies, it has been shown that the introduction of NF containing NPs of zinc oxides into the composition of the compound improves the thermo-oxidative stability of the obtained nanocomposites.

Numerous experimental data on the mechanical, strength, relaxation, and other properties of polymer-polymer and polymer-filler mixtures are explained within the framework of the concept of the presence of an interphase layer [18]. The properties of polymer composites are noticeably influenced by the supramolecular structure of the polymer (size of spherulites, degree of crystallinity, presence of C=O groups and various branches, etc.) and interfacial interaction at the interface [19].

The metal-containing nanoparticles used in this work, being located at the interface of the interphase layer of the structural elements of PP and PE, contribute to the formation in the melt of a composition of heterogeneous nucleation centers, which, in the process of stepwise cooling of the nanocomposite, contribute to an increase in crystallization centers, leading in general to an improvement in the crystallization process and the formation of a relatively small-spherolite structure.

The results show that small amounts of nanofiller (0.5–1.0 wt %) introduced into the polymer play the role of structure-forming agents—artificial nuclei of crystallization, which contributes to the formation in the polymer of a small-spherolite structure characterized by improved physicomechanical, rheological, and thermal properties of the obtained nanocomposite [20].